This invention relates to a switching device and arose because of the need for a thyratron switch capable of high speed switching of very large currents.
U.K. Patent Specification No. 1344802 (Philips) describes a semiconductor switching device comprising a number of switches 7, 9, 11 and 13 connected in parallel. A switching impulse is applied to terminal B.sub.1, through diode 17, line 18 and resistors 20, 21, 22, 23 to the control electrodes of the switches to switch them on simultaneously. Should one switch (e.g. switch 7) operate before the others it passes a current from terminal A.sub.1, through resistor 6 and a transformer primary 15 to terminal C.sub.1. This induces an impulse in transformer secondary 16 which passes through a diode 19 to reinforce the switching impulse on line 18 thereby expediting operation of the other switches.
The diode 17 is needed to prevent the impulse in the transformer secondary being short circuited to terminal B.sub.1. The diode 19 is needed in order to prevent the switching impulse from terminal B.sub.1, being short circuited through transformer secondary 16 to terminal C.sub.1.
This known circuit is not practicable for switching very high currents since the number of individual semiconductor switches would be prohibitive. For high currents it is necessary to use thyratrons but thyratrons cannot be connected in parallel in the manner described for semiconductor switches in the aforementioned Philips Specification. This is because a thyratron may develop for a short time, immediately after switching, a very high potential on its control grid. This would destroy diodes like those shown at 17 and 18 in the Philips Specification. The obvious way of dealing with this problem would be to replace the diodes 17 and 19 with suitable alternative diodes capable of holding off the aforementioned potential. This however would introduce a serious cost problem.